With the progress of advanced information society in recent years, there are strong requests for speeding up in a transmission rate and improving accuracy in transmission of information communication equipment and an examination/inspection apparatus of semiconductor devices applied to the equipment. Under the circumstances, speeding up in a transmission rate and improving accuracy are also requested in a coaxial cable and a coaxial cord.
When typical electric characteristics required for a coaxial cable will be mentioned, they are as follows.Propagation delay time(Td)=√ε/0.3(ns/m)Relative transmission rate(V)=100/√ε(%)Characteristic impedance(Zo)=60/√ε·LnD/d(Ω)Electrostatic capacity(C)=55.63ε/LnD/d(pF/m)
where ε: specific inductive capacity of an insulating member, D: outer diameter of the insulating member (inner diameter of an outer conductor), and d: conductor outer diameter (outer diameter of an inner conductor).
From the above description, it is found that transmission characteristics of a coaxial cable are influenced by the specific inductive capacity and the outer diameter of an insulating member, and the outer diameter of an inner conductor. It is understood that concerning the specific inductive capacity of an insulating member, the smaller value thereof result in the better transmission characteristics, and that concerning outer diameters of an inner conductor and an insulating member, its transmission characteristics are remarkably influenced by a ratio and dispersion. Particularly, as to characteristic impedance and electrostatic capacity, it is understood that ideal is in that a specific inductive capacity of an insulating member is small and the dispersion thereof is less, and in that dispersion of outer diameters (an inner diameter of shield layer) and the like of an inner conductor and an insulating layer and the outlines thereof are formed so as to have a generally more complete round sectional cylindrical shape.
In this respect, however, a conventional coaxial cable involves the problems described in the following paragraphs (1) to (3).
(1) An inner conductor applied to the coaxial cable is a silver-plated soft copper wire of AWG 20 to 30, or a twisted conductor obtained by twisting them. However, a diameter tolerance of a silver-plated soft copper wire is ±3/1000 mm, while when a twisted conductor is obtained by, for example, twisting seven strands, an outer diameter tolerance of the resulting twisted conductor becomes ±3×3/1000 mm. Due to the results mentioned above, when a preparation of a cable is intended within ±tolerance of the outer diameter, it becomes a remarkable variation factor in the above-mentioned characteristic impedance, electrostatic capacity and the like. This result becomes the higher with appearance in the thinner inner conductor.
(2) Concerning a foam insulating member applied to the coaxial cable, it is intended at present to reduce a transmission time and attenuation by making a porosity (ratio of forming) to be 60% or more to increase air gaps, whereby a specific inductive capacity (ε) is made to be 1.4 or less in order that a propagation delay time of the cable is made to be smaller as less as possible thereby to expedite a transmission rate. A member prepared by winding a porous tape member made of polytetrafluoroethylene (PTFE) (for example, those described in patent literary documents 1 and 2) on the outer periphery of an inner conductor, and calcining the inner conductor thus wound at the time or after winding the porous tape member is used as an insulator material having a porosity of 60% or more and a specific inductive capacity of 1.4 or less, and there is a polyethylene tape member having a weight average molecular weight of five million or more (for example, that described in patent literary document 3) as the other porous member than that mentioned above.
Patent literary document 1: Patent Publication No. 42-13560
Patent literary document 2: Patent Publication No. 51-18991
Patent literary document 3: Patent Application Laid-Open No. 2001-297633
However, these insulating layers exhibit remarkable dispersion in their thicknesses and porosities in view of properties of a porous tape member, so that improvement is strongly demanded in stability of transmission characteristics of a coaxial cable. Particularly, in a coaxial cable wherein an inner conductor size is made to be a thin diameter conductor of AWG 24 or more and a characteristic impedance value is made to be 50 Ω, dispersion in thickness, outer diameter, porosity, calcination and the like become remarkable drawbacks in view of eliminating dispersion of transmission characteristics for intending to realize stability.
Moreover, since the insulating layer is constituted by winding a porous tape member on the outer periphery of an inner conductor, irregular outline due to gap portions and overlapping of the tape appears in the overlapped portions of the tape member on the outer periphery of the conductor, whereby dispersion in specific inductive capacity and its outer diameter increases remarkably.
Furthermore, since the insulating layer is constituted by winding of a porous tape member having a very low mechanical strength, it is required to significantly decrease tension of the tape member as less as possible in order to eliminate elongation and breaking of the tape member itself at the time of winding thereof and also elongation and disconnection of a superfine inner conductor. Because of the situation, an irregular outline and dispersion in its outer diameter become more remarkable, besides a degree of adhesion of the tape member is very weak with respect to an inner conductor, so that dispersion in its specific inductive capacity and its outer diameter expands further.
In addition, since a specific inductive capacity is reduced for the primary objective of decreasing a propagation delay time of a cable as less as possible to increase a transmission rate in the insulating layer, there still remains such drawback in mechanical strength that the coaxial cable is difficult to assure a structural dimension as a coaxial cable due to mechanical stress such as bending, torsion, pressing, sliding and the like which will be received by the coaxial cable. The most remarkable disadvantage is in that it is difficult to maintain the outer diameter of an insulating member in a predetermined outer diameter to eliminate its dispersion, and further to form the outline of the insulating member in a cylindrical shape.
(3) In such type of conventional coaxial cables as mentioned above, a member prepared by either winding a plastic tape member either surface of which contains a metallic layer such as a copper layer on the outer periphery of an insulating member, or including the plastic tape member lengthwise on the insulating member; a member constituted by a silver-plated soft copper wires having an outer diameter tolerance of ±3/1000 mm JIS standard or a braid member of the silver-plated soft copper wires braided with tin-plated soft copper wires; or a member in combination of the tape member and the braided member is used as an outer conductor participated remarkably in transmission characteristics of a coaxial cable.
However, in the member obtained by winding the tape member or including the tape member lengthwise, flexibility of the cable is insufficient, and thus, its outer conductor is easily broken by mechanical stresses such as flexure, and torsion which will be added to the cable, whereby functions as an outer conductor cannot be achieved. The braided member of silver-plated soft copper wires involves such problems that since slippage of silver is small, frictional force due to contact in the silver-plated soft copper wires with each other increases, so that movements in respective strands constituting the braided member disappear, whereby flexibility of the cable is lost, resulting in deformation of an insulating layer, and characteristic impedance values vary. Besides, it cannot decrease influences by mechanical stresses, and thus a life of the cable becomes shortened.
When the braid member of tin-plated soft copper wires is used under a high temperature (80° C. or higher), copper diffuses into a tin-plated layer, and production/growth of tin whiskers are accelerated due to diffusion stress. When the whiskers grow remarkably, they burst through an ultrathin insulating member, and as a result, there is case where the whiskers short-circuit with its inner conductor. Moreover, the above-described respective outer conductors are formed on the periphery of an insulating member which contains irregular outline and dispersion in its outer diameter as mentioned in the explanation of the insulating member in the paragraph (2) Accordingly, inner and outer parts of an outer conductor are irregular and dispersion in its outer diameter remains remarkably, so that a number of gaps are contained in between the outer conductor and the insulating layer, whereby a varying factor of specific inductive capacity still remains.
The present invention has been made in view of the above-described problems, and an object of the invention is to provide a foam coaxial cable which can speed up a transmission rate, improve accuracy in characteristic impedance values, make flexibility of a cable better, and assure a predetermined mechanical strength by decreasing mechanical stresses such as flexure, torsion, pressing, and sliding, even when such mechanical stresses are added to the cable, besides it can also reduce variations in characteristic impedance values.
Furthermore, another object of the present invention is to provide a method of manufacturing a foam coaxial cable which can intend to improve accuracy in characteristic impedance values between an inner conductor and an outer conductor, and stabilize a secondary shaping step by such a manner that a highly foamed insulating layer of the coaxial cable containing a foam insulating layer (a degree of foaming of 60% or more) to which a porous tape member is applied and the outer conductor are subjected to secondary shaping, whereby thicknesses and outer diameters of them are uniformized, and the outline of which is made to be a generally complete round shape.